The present invention relates to an improvement in a power transmission unit and, more particularly, to a viscous coupling apparatus for use in motor vehicle drivetrain applications.
Conventionally, viscous couplings have been incorporated into motor vehicle drivetrain arrangements for transmitting power in response to the viscous shear characteristics of the fluid confined therein. In general, viscous couplings have a first plurality of clutch plates fixedly splined to a first rotatable member and interleaved with a second plurality of clutch plates fixedly splined to a second rotatable member. The first and second rotatable members define a sealed chamber within which the interleaved clutch plates are confined. The sealed chamber is charged with a viscous fluid for filling the spaces between the interleaved clutch plates. In operation, the viscous coupling transmits only a small amount of torque when the rotational difference between the first and second rotatable members is small, while transmitting a relatively large torque when the rotational difference is portionally larger due to the viscous shear resistance of the viscous fluid between the interleaved clutch plates.
Modernly, viscous couplings are used in numerous power transmission applications such as four-wheel drive transfer cases, differentials and limited-slip intra-axle and inter-axle devices. Examples of transfer cases which incorporate a viscous coupling therein are disclosed in U.S. Pat. No. 4,031,780 to Dolan and U.S. Ser. No. 437,066 filed Nov. 16, 1989 to Frost. For purposes of illustration, FIG. 1 is an enlarged view of viscous coupling apparatus 10 which is substantially identical to that shown in FIG. 2 of the above-mentioned U.S. patent application to Frost. Viscous coupling 10 includes an outer drum assembly 12 and an inner drum housing 14 which define a fluid chamber 16 therebetween. In general, outer drum assembly 12 defines a first rotatable member which is coaxially arranged with respect to inner drum housing 14 which defines a second rotatable member. More particularly, outer drum assembly 12 includes a cylindrical outer drum housing 18 and annular front and rear end cover plates 20 and 22, respectively, which are adapted to close the opposite ends of cylindrical outer drum housing 18. Front cover plate 20 includes a forwardly projecting annular neck section 24 formed within an internally splined opening 26 concentrically disposed about a central axis 28. Internally splined opening 26 is fixedly secured to external splines formed on a gear member, partially shown at 30, for transmitting drive torque to a first output shaft (not shown) of the transfer case for driving a front propeller or drive shaft in a known manner. Likewise, inner drum housing 14 has internal splines 32 adapted for meshed engagement with external splines 33 of a central shaft 34 extending concentrically therethrough for rotation about central axis 28. Central shaft 34 defines a second output shaft of the transfer case for transmitting drive torque to a rear propeller or drive shaft.
A central radially outwardly stepped portion 36 of inner drum 14 is formed to include external longitudinal splines 38 upon which a first plurality of "inner" resistance clutch plates 40 are fixedly secured. Inner clutch plates 40 are flat annular ring-shaped members having splines 42 formed on their inner peripheral bore which are retained on, and non-rotatably engage, inner drum housing external splines 38. A radially inwardly stepped portion 44 of outer drum housing 18, generally concentrically aligned with stepped portion 36 of inner drum housing 14, has internal longitudinal splines 46 formed thereon upon which a second plurality of "outer" resistance clutch plates 48 are mounted. Similarly, outer clutch plates 48 are flat angular rings having splines 50 formed at their outer periphery which are retained on, and non-rotatably engage, outer drum internal splines 46. Furthermore, inner and outer clutch plates 40 and 48, respectively, are alternately interleaved within fluid chamber 16 with an outer clutch plate 48 positioned adjacent each end cover plate 20 and 22.
As in most conventional viscous couplings, inner clutch plates 40 are axially separated by a plurality of annular ring spacers 52 that concentrically surround inner drum longitudinal splines 46. Ring spacers 52 have a predetermined width (i.e. diameter) which is selected to provide a desired "pack" spacing between adjacent inner clutch plates 40 and, in turn, between interleaved clutch plates 40 and 48. Generally, ring spacers are not also used with outer clutch plates 48 and thus allow axial sliding movement of outer clutch plates 48 between adjacent inner clutch plates 40 along splines 46. In addition, fluid chamber 16 is filled with a predetermined quantity of a highly viscous fluid, such as a silicone oil, admitted by way of a filling hole 54 and closed by a seal plug 56. To hermetically seal the viscous fluid within fluid chamber 16 upon relative rotation of inner drum housing 14 and outer drum assembly 12, an elastomeric annular seal 60 and back-up ring 62 are disposed in longitudinally spaced annular recesses 66 that are formed on the inner periphery of front and rear cover plates 20 and 22, respectively.
As is known, viscous coupling 10 is operable to proportionally modify the torque division between central shaft 34 and gear member 30 and, in turn, between the first and second transfer case output shafts in response to the relative rotational speed differential existing between inner drum housing 14 and outer drum assembly 12. Generally, during most driving conditions, inner drum housing 14 and outer drum assembly 12 will be rotating at a substantially similar speed (i.e. the vehicle is travelling on dry pavement). Where the driving conditions involve a slight differential in rotational speed, the viscous fluid will permit viscous shearing to accommodate the rotational differences by a allowing slip. However, as the rotational speed differential and viscous shearing rate increase, the apparent viscosity of the fluid will decrease resulting in a stiffening of the fluid and a transmittal of torque from the clutch plate exhibiting a higher rotational speed. Thus, as the viscous shearing rate increases, viscous coupling 10 becomes increasingly rigid for thereby transmitting an increased amount of torque.
As is known, the "pack" spacing between interleaved inner plates 40 and outer clutch plates 48 is a critical design parameter for developing the desired viscous shear characteristic (i.e. torque vs speed differential) for a given motor vehicle drivetrain application. While conventional viscous couplings, such as those described herebefore, generally perform satisfactorily variations in dimensional stack-up tolerances and assembly tolerances may cause undesirable variations in the viscous shear characteristics and, in turn, the viscous coupling's torque transmission performance. More particularly, it has been discovered that the outermost (i.e. left and right) ring spacers 52 may "fall off" splines 46 on outwardly stepped inner drum portion 36 due to the aforementioned tolerance variations which detrimentally effects the desired clutch plate "pack" spacing. Upon such an occurrence, viscous coupling 10 is not capable of generating the desired torque modification performance characteristics such that the apparatus is largely inoperative for its intended purpose.